Field devices, such as radar level gauges, are suitably used for measuring the level of products such as process liquids, granular compounds and other materials. An example of such a radar level gauge can include a microwave unit for transmitting microwaves towards the surface and receiving microwaves reflected by the surface, processing circuitry arranged to communicate with the microwave unit and to determine the level based on a relation between transmitted and received microwaves, an interface for connecting said processing circuitry externally of the radar level gauge, and a power management circuitry providing the microwave unit and the processing circuitry with operating power.
More particularly, the distance to the surface of the product is generally determined based on the time between transmission of an electromagnetic signal and reception of the reflection thereof in the interface between the atmosphere in the tank and the product contained therein. In order to determine the actual filling level of the product, the distance from a reference position to the surface is determined based on the above-mentioned time (the so-called time-of-flight) and the propagation velocity of the electromagnetic signals.
In order to accurately determine the time-of-flight of the electromagnetic signals, the received signals must have sufficient amplitude or power so that they can be detected and analyzed. Hence, it is important that propagation losses are kept to a minimum. Propagation losses can for example be the result of scattering or damping of the signal, both leading to a reduction in power of the signal reaching the receiver/transceiver. Propagation losses may occur both for transmission line probes, waveguide structures and radiating antennas.
Contaminating materials adhering to an antenna may cause scattering of the signal as the geometry of the antenna is altered. The adherence of contaminants on an antenna may also lead to damping of the signal in the case that the signal has to propagate through contaminating material on the antenna.
Addressing the problem of contamination is particularly relevant in tanks for storage of petroleum products which may easily stick to the surface of the antenna, thereby leading to a deterioration of signal quality.
An attempt to solve the abovementioned problem of contamination is disclosed by U.S. Pat. No. 7,707,881 relating to a fill level sensor with an antenna having an anti-adhesive coating. An objective of U.S. Pat. No. 7,707,881 is to achieve a non-wettable and self cleaning surface preventing any materials from sticking to the surface of the antenna. According to U.S. Pat. No. 7,707,881, this objective may be is achieved by forming a microsmooth surface having anti-adhesive surface properties.
In particular for some combinations of tank content and environmental conditions, however, it has turned out that anti-adhesive coatings such as that described in U.S. Pat. No. 7,707,881 are not capable of preventing adhesion of tank content to the antenna, resulting in loss of performance.